Arsenic is a naturally-occurring element found in ground and surface water, and is present in high concentrations in many parts of the world. Also, there are many locations where ground water has been contaminated with arsenic from industrial activities such as mining operations, waste pile run off and pesticide manufacturing.
It has long been known that arsenic is a highly toxic substance, a suspected carcinogen, and can be deadly in pure form. The long-term effects of consuming water with naturally occurring high levels of arsenic have been the subject of numerous studies. It has been found that the so-called chronic arsenic poisoning can cause thickening and discolouration of the skin, cancers of the liver, kidney and skin, and loss of circulation in the extremities causing a gangrenous-like condition known as blackfoot disease.
Due to the toxic and carcinogenic nature of arsenic, government agencies have established a maximum acceptable concentration of allowable arsenic in drinking water. In Canada, Health and Welfare Canada has set a maximum acceptable concentration of 25 .mu.g/L in drinking water. The World Health Organization has established a maximum acceptable concentration of 10 .mu.g/L. Currently, the USA limit is 50 .mu.g/L, but the USA's Environment Protection Agency is in the process of revising this limit downward, perhaps to a standard as low as 10 .mu.g/L. It is known that several States have already reduced the limit to 10 .mu.g/L.
Several methods have been used in the past for removing arsenic from water. Existing surface water treatment plants employing conventional treatment trains such as lime softening, and coagulation/flocculation/filtration have shown arsenic removal abilities as a side effect. Advanced technologies such as ion exchange, activated alumina, and membrane processes such as reverse osmosis, have had much less testing but have shown good potential under certain conditions. However, it has been demonstrated in previous research that the background water quality matrix strongly influences arsenic removal. For examples, alkalinity affects coagulation processes; sulfates affect ion exchange and membrane processes; activated alumina performance declines with increasing pH and fluoride concentration, and co-precipitation with iron is inhibited by high chloride concentration.
In addition to the possible presence of inhibitory substances in water, other difficulties associated with the removal of arsenic from ground water include high cost, complexity, and method of use. For examples, lime softening or coagulation/flocculation/filtration plants are expensive and they require a relatively high degree of operator attention. These plants are also known to create large quantities of residuals which can pose disposal problems. Activated alumina and ion exchange media are expensive to manufacture. Membrane filtration plants are also expensive to build, can be technically challenging to operate, and often result in wastage of over 50% of the water supply. In some cases it has been found that for every 100 litres entering the plant, often less than 50 litres of high-purity water is produced, with the remaining being wasted.
Recently, research has been carried out regarding the use of iron-oxide coated sand as filtering media. The iron-oxide coated sand has been shown to efficiently remove arsenic from ground water. However, this product has not been developed on a large scale, primarily due to the fact that the existing formulation requires a complex and expensive preparation including the baking of an iron-oxide coating onto the sand particles. To date, this preparation procedure has been limited to production of small quantities for laboratory experimentations.
Similar research has been carried out with respect to the use of iron-oxide impregnated porous support materials as filtration media. For example, Canadian Patent 1,067,627 issued to Gerald D. Lutwick on Dec. 4, 1979 teaches a method and apparatus for the removal of arsenic from water by passing water containing arsenic over a porous support material which is impregnated with ferric hydroxide. The Lutwick patent provides two examples of filtering materials for removing arsenic from ground water. In the first example the filtering material was impregnated with 4.4% ferric ions as Fe(OH).sub.3, and in the second example, the filtering material was impregnated with 0.97% ferric ions as Fe(OH).sub.3. The tests were carried out at a water pH of 4.7 in the first case, and with a water pH of 3.9 in the second case. The Lutwick patent further teaches that arsenic removal is preferably carried out with the water at an optimum pH of 4.4.
The Lutwick patent also teaches that in some applications, the filtering material may be regenerated. The Lutwick patent is silent with regard to a method of regeneration of the filtering material or whether regeneration involves re-impregnation with Fe(OH).sub.3.
As such, it will be appreciated that there continues to be a need for a filtering medium capable of use with well water as it comes out of the ground, without pH adjustment before or after the filtration process. There continues to be a need for a filtering medium from which arsenic can be desorbed and washed out, and the adsorption capacity of which can be restored to an effective level without having to re-impregnate ferric ions therein.
Further, it is believed that there continues to be a need for an arsenic filtering medium which is relatively inexpensive, effective over a wide range of water chemistry and capable of being used on residential wells as efficiently as in industrial, commercial and municipal installations.